Thicker oil (or oil thickening schmutz) is NEVER the answer. In fact a heavier oil, especially now that we are going into the cooler weather, will accelerate wear. I lost an engine due to STP+20w/50 one cold morning.
Sure we've all thrown STP into a sick automatic, or filled differentials with bacon grease and sawdust to get something sold. But this is only a "fix" and not a REPAIR. Verify your actual pressure and decide what to do then. If you have at least 5 psi at idle, and 30 or so going down the road, you should be fine for a while on the street.
BTW 81k is pretty low mileage for a 318 to be causing oil pressure problems, unless it seldom saw an oil change, or was used for short trips.